The signal fire is atop a tall tower that is separated from the rest of the mainland. To get there, you need to stand on the destroyed bridge, shoot an arrow towards the bundle of ropes and, at the other side of the chasm, zip down the line.
Start climbing around the spire. As soon as you get to the spot shown in the screenshot above, press the B button to make Lara let go of the rock and, a fraction of a second later, press X button to make her jab her hatchet into the rock below. Climb to the right.
To get to the top, you will have to take several double-jumps.
Before you reach the signal fire stack, you will get to one of the lower levels. Here, you find the ingredient thanks to which you can use fire arrows.
The final phase of the climb consists in jumping over towards the wooden lift. There are smears of white paint on its edges, so you can easily grab on to them.
While atop the lift, look up, select flame arrow and shoot it towards the stack.